Conventionally, building-block-type programmable logic controllers constructed of multiple units have been used in the field of factory automation. As to programmable logic controllers, a technique has been proposed, in which a connection state of a bus between a programmable logic controller and an external device is detected by a hardware circuit and resultant information is stored in a CPU (refer to, for example, Patent Literatures 1, 3 and 4). Further, for example, in Patent Literature 2, a technique is proposed, in which states such as disconnection or short-circuiting of connection between an input slave unit of a programmable logic controller and an external device are detected, and the place where a communication anomaly exists is identified.